1. Field of the Invention
The invention relates to test instruments, and more specifically to test instruments for the testing of local area network cables.
2. Description of the Related Art
High-speed data transmission cables are often an integral part of computer networks and telecommunication systems. A local area network (LAN), for example, as illustrated in FIG. 1 will usually include a number of individual computers and peripheral devices 60 communicating to one another through data cables 62. The performance of the network can be affected by the quality of the data cables 62, therefore it is important to test the cables to ensure the network is configured correctly. A number of companies have developed hand-held test instruments that allow network installers to quickly and accurately test the cables used to construct the LAN. These test instruments must be electrically reliable under field cable installer and cable test conditions. It is desirable for the instruments to allow for ready replacement in the field of the cable test connector, which, in the case of LAN cables is the RJ-45 connector. The instruments must be mechanically rugged, reliable, and professional in appearance.
LAN""s are generally constructed of a twisted wire cable containing a number of twisted wire pairs. Existing hand-held test instruments will run a battery of tests to evaluate the performance and connections of each of the twisted wire pairs within each cable. Representative tests performed by the test instruments include cross talk, attenuation, length of the cable, and noise. Each of these tests relies on the measurement of a high frequency (RF) signal which is transmitted through the wire pair. The actual RF measurements in the test instrument typically take place on an RF measurement Printed Circuit Board (PCB).
Some known test instruments separate the test hardware into a main unit and a performance/adapter module such as the instrument disclosed by Moser et al, U.S. Pat. No. 5,677,633, entitled xe2x80x9cCable Test Instrument Having Interchangeable Performance Modules.xe2x80x9d The performance/adapter module is an interchangeable module having a compatible connector for attachment to the main unit and a cable connector for attachment to a cable being tested. The above described instrument configuration allows the test instrument to be connected to a variety of different LAN cables by simply replacing the performance/adapter module.
The prior art experiences limitations when the test equipment is separated into two pieces of hardware, as described above and illustrated in FIG. 2. The test equipment illustrated in FIG. 2 includes an adapter housing 70 and a main housing 72 designed to receive the adapter housing 70. The adapter housing 70 and the main housing 72 each include a mating connector. Upon insertion of the adapter housing 70 into the main housing 72, the mating connectors engage one another to form electrical and mechanical connections whereby electrical signals are communicated between the main housing 72 and the adapter housing 70. This connection is often inadequate to provide a reliable electrical and mechanical connection which is necessary for LAN testing. Due to the orientation of the connectors there may be misalignment when inserting the adapter housing 70 into the main housing 72 which could possibly damage the connectors. Moreover, misalignment between the mating connectors can also produce discontinuities in the electrical connection from one piece of the test equipment to the other.
A shroud may be used to lock the adapter housing 70 into the main housing 72. However, this configuration additionally requires a user to engage the connectors while being visually blocked from observing any potential misalignment between the two connectors. Furthermore, if the shroud does not adequately constrain the connector locations just prior to connector mating, the connectors may be damaged by any misalignment. When a straight-in connection orientation is used between the main housing 72 and the adapter housing 70, as illustrated in FIG. 2, strain can be placed on the mechanical connection from excessive movement or pull on the adapter if the adapter housing 70 is not locked securely into the main housing piece. As a result of this strain on the mechanical connection, the electrical connection between the adapter housing 70 and the main housing 72 may degrade over time and become unsuitable for providing accurate test measurements.
In addition to misalignment in the connectors, some prior art designs create a lever arm due to the orientation of the mechanical interface between the mating connectors of an adapter housing and of a main instrument housing when the adapter housing protrudes beyond the outer surface of the main housing. This lever arm puts additional strain on the electrical connection and may therefore affect the accuracy of the measurements being performed.
An additional problem experienced with designs where the test equipment is separated into two pieces is that the test signal must propagate through two printed circuit board""s (xe2x80x9cPCBxe2x80x9d) instead of a single PCB. These PCB""s are typically connected from one module to another with a right angle connector on each module. Bends in the propagation path of an RF signal greatly affect the signal properties. These approximate right angle bends due to the use of a right angle connector introduce discontinuities in the characteristic impedance of the connection in addition to the discontinuity of the connector itself.
As illustrated in FIG. 3, the use of a right angle connector 80 in a removable cable adapter assembly 82 and a mating connector 81 in a main instrument housing 86 in the prior art may introduce at least four right angle bends. Two of these right angle bends are at areas 88 and 90 on the removable cable adapter assembly 82. The first right angle bend 88 consists of horizontal board traces from a PCB 91 meeting the connector via a right angle. The second right angle 90 is either a full right angle or a short radius that is part of the connector 80. The right angles 88, 90 in the cable adapter assembly are mirrored in the main instrument housing by a third and fourth right angles 92, 94, adding to the signal measurement degradation.
In addition to the impedance disruptions caused by these bends in the signal path, the areas of the second and third right angles 90, 92 are not fully shielded. This leads to undesired cross talk between signal pairs. These errors contribute additional error in instrument test measurements.
It will be appreciated that the electrical and mechanical connection orientations of present test instruments in the field limit the accuracy and reliability of the tests performed on a cable. The electrical limitations become more apparent at higher test frequencies and the mechanical limitations lead to reliability issues. It would therefore be advantageous to develop a test instrument with improved electrical and mechanical connection orientations to provide a more secure connection between a main housing piece and an adapter housing piece and to increase accuracy of cable test measurements.
The systems and methods of the present invention have several features, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention as expressed by the claims which follow, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled xe2x80x9cDetailed Description of the Inventionxe2x80x9d one will understand how the features of this invention provide several advantages over traditional test instruments.
One aspect of the present invention is an apparatus for performing testing of local area network cables. The apparatus comprises an adapter module having a cable or connector located proximate to one end, a first printed circuit board connected to the adapter module to form a first wire connection, and a first connector connected to the first printed circuit board on a first surface of the first printed circuit board. The first connector includes a mating surface which is substantially parallel to the first surface of the first printed circuit board, and wherein there are substantially no bends in an electrical path from the first wire connection to the first mating surface. The apparatus further comprises a main instrument housing having an internal recess configured to receive the adapter module, a second printed circuit board connected to the housing to form a second wire connection, and a second connector connected to the second printed circuit board on a second surface of the second printed circuit board. The second connector includes a second mating surface which is parallel to the second surface, and wherein there are substantially no bends in an electrical path from the second wire connection to the second mating surface. The first connector engages the second connector on the first and second mating surfaces upon insertion of the adapter module into the main instrument housing.
Another aspect of the invention is a method of manufacturing a cable test instrument. The method comprises connecting a first printed circuit board having a first surface to a first connector having a first mating surface such that the first mating surface is substantially parallel to the first surface of the first printed circuit board. The method further comprises inserting the first printed circuit board and first connector in an adapter housing. The adapter housing is connected to a cable or connector located proximate to one end. The method further includes connecting a second printed circuit board having a second surface to a second connector having a mating surface such that the second mating surface is parallel to the second surface of the second printed circuit board. A main instrument housing is provided having an internal recess configured to receive the adapter housing. Finally, the second printed circuit board and the second connector are inserted in the main instrument housing such that the first connector engages the second connector on the first and second mating surfaces upon insertion of the adapter housing into the main instrument housing.